The present invention relates to an epitaxial layer structure grown on a graded substrate and a method of epitaxial growth on the graded substrate.
At present, mixed crystals of various semiconductor material such as GaAlAs, InGaAsP, InGaAlAsP, and ZnCdSSe are used as epitaxial layer for a semiconductor device, which is grown on a substrate of a similar mixed crystal. In the case of InGaAsP quarternary crystal, for example, it is possible to achieve a wide range of energy bandgaps from 0.36 eV to 2.25 eV by varying the composition of the mixed crystal from InAs to GaP. For this reason, the InGaAsP epitaxial layer is available for various optoelectric semiconductor devices such as laser diodes, light-emitting diodes, and photo detectors wherein the InGaAsP layer can have a desired wavelength ranging from the infrared region to the visible region.
Generally speaking, if a crystal having a lattice constant different from that of the substrate is grown on the substrate, a lot of misfit dislocations are introduced from its interface into a grown layer, which undermines the crystal quality of the grown layer. Consequently, the surface condition of the grown crystal is also adversely affected, making it impossible to obtain usable crystals.
Since the epitaxial layer should be lattice-matched to the substrate, possible bandgap range of the epitaxial layer depends on the composition of the substrate. For instance, when the InGaAsP layer is grown on an InP substrate with a lattice match condition, the bandgap range is restricted to 0.75 eV to 1.28 eV. When the GaAs substrate is used, the bandgap is restricted to the range between 1.48 eV and 1.88 eV. To expand the possible bandgap range, ternary or quarternary substrate is required instead of conventional binary substrate. However, it is very difficult to obtain a bulk single crystal substrate for the ternary or quarternary system.
To this end, so-called graded substrate is commercially available. The graded substrate comprises a binary bulk single crystal substrate, a graded composition layer formed on the binary substrate, and a constant composition layer formed on the graded composition layer. A typical example of such a commercially available GaAs.sub.0.61 P.sub.0.39 wafers consists of a 30 .mu.m-thick graded composition GaAsP layer on a GaAs substrate and a 20 .mu.m-thick constant composition GaAs.sub.0.61 P.sub.0.39 layer on top of the wafer. Using this wafer as the graded substrate for growing a desired ternary or quarternary epitaxial layer such as InGaP or InGaAsP thereon, however, the grown surface has a so called cross-hatched pattern and a number of small surface defects appear when InGaP and InGaAsP layers are grown under the lattice matched condition on the GaAsP graded substrate. To achieve a good semiconductor device, therefore, it is necessary to eliminate the above mentioned deffects.
Accordingly, an object of the present invention is to provide a crystal structure having a graded substrate and an epitaxial layer grown on the graded substrate and a method making the same with a good surface condition and crystal quality by eliminating the aforementioned conventional drawbacks.
The present invention is featured in that an intentional lattice mismatch is introduced between the epitaxial layer crystal and the graded substrate surface by the value of .DELTA.a/a=1.times.10.sup.-3 to 5.times.10.sup.-3 (.DELTA.a: a difference in the lattice constant between the top layer of the graded substrate and the grown layer thereon, a: the lattice constant of the top layer of the graded substrate) so as to alleviate a strain resulting from a change in the lattice constant within the graded substrate. When the lattice constant in the graded layer is increased from a lower layer (bulk substrate) toward the top layer (the constant layer), the lattice constant of the epitaxial layer should be selected to be smaller than that of the constant layer. On the other hand, when the lattice constant of the graded layer is decreased from bulk substrate toward the constant layer, the lattice constant of the grown layer should be selected to be larger than that of the constant layer.